In the middle ages (I maybe wrong about this...) they had earnest discussions about how many angels could dance on the head of a pin. (You may substitute any other un-verifiable, un-testable topic you like)

I think efforts to plan technologies more than a decade or so ahead are doomed to be irrelevant. We need to get out of the cradle before we start wondering what running shoes we will be using.

Lets work out what steps we can take in the immediate future to get to the point where we can sustain colonies off-earth first. Then we will find that we have enormous resources available right here in our own solar system. By the time we need to start to worry about system resource depletion we will already have a whole range of new technologies to consider for the next step.

Gourdhead I would suggest that your efforts are better spent on finding a more immediate problem... the far future will likely look a lot different than you or I imagine it to be.